Although much-criticized trans fats raise levels of "bad" cholesterol, they don't appear to have a lasting impact on blood sugar levels, according to a U.S. study.
Researchers, writing in The American Journal of Clinical Nutrition, found that both blood sugar and insulin, the hormone that keeps blood sugar levels in check, were similar regardless of how much trans fat people ate.
The link between trans fats and high cholesterol levels is widely accepted, but there has been a lack of clarity on the effect on blood sugar control, which is involved in diabetes.
"Although evidence from cohort studies has suggested that trans fatty acid (TFA) consumption may be associated with insulin resistance and diabetes, randomized placebo-controlled trials have yielded conflicting results," wrote lead researcher Christos Mantzoros of Harvard Medical School in Boston.
Trans fats, technically known as trans fatty acids, are found in animal products and chemically processed vegetable oils. In response to studies linking high consumption of the substances to an increased risk of heart disease, the U.S. Food and Drug Administration has required food makes to disclose trans fats on nutrition labels.
Some cities and states have banned them in restaurants or schools.
Montzoros and his colleagues pooled the results from seven experiments, including 208 people.
In five of the studies, the participants' blood sugar, insulin and cholesterol levels were monitored for several weeks under a diet of high trans fat consumption, and again for a few weeks when the trans fats were substituted for other fats, such as palm or soybean oil.
Two of the studies compared people who ate a diet that included trans fats to others who ate a diet without trans fats.
There were no changes in blood sugar or insulin levels during the times when people ate trans fats, compared to when they ate the other fats," Mantzoros's group reported.
However, the researchers found that during the trans fat-eating weeks, "good" HDL cholesterol went down and "bad" LDL cholesterol went up.
"They saw what you would expect to see" regarding cholesterol, which shows that the studies were well done, said Mark Pereira, an expert in public health and nutrition at the University of Minnesota in Minneapolis.
Pereira, who was not involved in the study, said it isn't definitive proof that trans fats can't influence blood sugar levels.
Although several weeks is enough time to see an effect on cholesterol, he said, a potential impact on metabolism might not show up until later.
"If you're going to control weight and switch around fats in the diet, it might take a lot longer, because these fatty acids are being gradually incorporated over time into tissues in the body," he added.
But even if trans fats do have an impact on blood sugar control, Pereira said, it's becoming a moot point as the amount of trans fats people eat in the United States has diminished considerably.
(Reporting by Elaine Lies)